1. Field of the Invention
The present invention concerns a device using friction in a liquid medium such as, in particular, a clutch or brake disc, notably for an automatic gearbox or associated therewith, or such as a locking clutch, commonly referred to as a lock-up clutch, or such as a gearbox synchronisation ring or cone which carries a friction lining with a flat annular shape overall intended to equip a device using friction in a liquid medium.
2. Description of Related Art
The friction lining is in the form of a flat ring or a truncated cone and the device which is equipped therewith is a clutch or brake disc, notably for an automatic gearbox and associated therewith, functioning in oil, or a manually or robotically controlled gearbox synchronisation ring or cone also operating in oil, such a device being installed on board a motor vehicle.
Naturally the device can have several clutch or brake discs as described in the document U.S. Pat. No. 5,566,302.
The invention concerns for example a device equipped with a friction lining of the type produced from a friction material consisting of a mat of fibres impregnated with a thermosetting resin and whose manufacturing method includes a final curing step, associated with an operating of moulding or overmoulding, during which the annular blank of the lining, previously produced from the mat of impregnated fibres, is pressurised.
The composition and structure of a friction material consisting of a mat of fibres impregnated with a resin of the thermosetting type are described in detail in the document WO-A-95/26.473.
This document mentions notably the structural advantages and the advantages in use of such a type of material compared with the materials of the state of the art, and notably compared with the material of the xe2x80x9cpaperxe2x80x9d type which consists essentially of cellulose fibres obtained by wet process in accordance with a normal paper manufacturing method, that is to say by dispersing cellulose fibres in an aqueous solution containing a resin, and then hydroextraction and drying.
Reference can also be made to the detailed content of this document in the context of the use of such a material for producing a friction lining on a device in accordance with the teachings of the invention.
This type of non-woven material, using a dry process, compared with the wet process used for obtaining a material of the paper type, has the structural advantage of being porous and including large porosities between 50 and 500 micrometers which communicate with each other, whilst the papers have small porosities of between 5 and 50 micrometers which do not communicate.
The concept of communicating porosity is a relative concept, that is to say it must be assessed with respect to the liquid in which the friction lining is immersed, that is to say in particular the oil of a transmission.
This is because oil does not pass through a material of the paper type whilst it does pass through the porous friction material produced in accordance with the teachings of the document WO-A-95/26.473.
The porosity of this type of material thus permits circulation of the oil actually inside the friction lining, notably from its inside diameter to its outside diameter and vice versa, which makes it possible to very effectively discharge the heat produced during the friction of the lining and thus to prevent temperature rises which might degrade the friction material, the oil or the system in general in which the clutch equipped with such a lining is incorporated.
Such an advantageous characteristic is of particular interest in controlled or continuous slip systems and in clutches.
On the other hand, this internal circulation of oil in the lining can be harmful in systems such as devices known as lock-up devices in which the friction lining must also fulfil a role of sealing joint in order to prevent the appearance of back pressures in the lock-up chamber resulting in a fall in the torque transmission capacity.
The document FR-A-2.760.504 has already proposed an improvement to a friction lining having recourse to a material of the type mentioned above in order to be able to use this type of material in different types of systems, perfectly controlling the flow of liquid.
This document proposes a friction lining of the type in which the method of manufacturing the friction lining includes a step of moulding, or overmoulding, and curing in a mould under pressure, and of the type which includes areas with different controlled porosities.
In accordance with the teachings of this document, being able to control the porosity of the friction material at all points on the lining, or in given areas of the latter, makes it possible to control the flow of oil inside the lining, or even to eliminate it, thus preserving the tribological properties of the friction lining.
According to other teachings of this document, the lining has at least one annular area with a lower controlled porosity, that is to say a density greater than the mean porosity of the body made of friction material constituting the lining, the annular area with controlled porosity consisting for example of a peripheral bevel arranged at one of the peripheral edges, internal or external, of the lining, or being obtained by virtue of a concentric annular insert, notably with an impermeable structure, whose thickness is less than the mean thickness of the flat annular lining, the annular portion opposite the material constituting the friction lining having a controlled porosity less than the mean porosity of the friction material.
This solution is notably satisfactory in the case of so-called lock-up devices in which the area of lower porosity makes it possible to xe2x80x9cblockxe2x80x9d the circulation of oil inside the lining.
In the document EP-A-0867,634 the variation in porosity is obtained by means of a porous support. This solution is not economical because of the constitution of the carbon-based support.
The aim of the invention is to propose an economical solution for improving the circulation of oil in the friction lining and to improve the distribution of the stresses and deformations in the friction material.
To this end, the invention proposes a device of the type mentioned above, having a support impermeable to liquid, notably metallic, on which an interface surface forms a support for a friction lining, produced from a friction material which has a porous structure with communicating cells and which has areas with different controlled porosities, and of the type in which the method of manufacturing the friction lining includes a step of moulding under pressure, characterised in that the interface surface of the support has at least one groove in which there extends the friction material moulded under pressure in order to constitute an area of controlled porosity greater than the mean porosity of the body made from friction material constituting the lining, notably in order to form a channel for the oil to circulate inside the friction lining.
By virtue of the invention a better connection is obtained between the friction lining and its support. A better mechanical strength is thus obtained, notably against the effects of centrifugal force. Good draining of the oil is obtained.
According to other characteristics of the invention:
the mean depth of the groove is between 10% and 50% of the mean thickness of the flat annular lining;
the groove is such that it has a section, transverse to its general direction, which is perpendicular to the direction of shearing of the friction material during use of the device;
at least part of the interface surface is not parallel to the active friction surface of the lining;
the support is overall in the form of a flat ring and in that the interface surface has a frustoconical profile and in that the active friction surface is not parallel to the frustoconical interface surface so as to constitute an annular area with a controlled porosity less than the mean porosity of the body made of friction material constituting the lining;
the interface surface is formed by the bottom of a wide annular groove formed in a lateral face of the flat annular support;
the frustoconical interface surface has at least one narrow groove in which the friction material moulded under pressure extends;
the friction material consists of a mat of fibres impregnated with a thermosetting resin;
the size of the communicating cells is between 50 and 500 micrometers;
the porosity of the friction material is less than 60%;
the active friction surface of the lining has a network of grooves affording better circulation of oil between the friction lining and a reaction surface with which the device cooperates.